1. Field of the Invention
Embodiments of the present invention generally relate to position location systems. More particularly, the invention relates to a method and apparatus for adjusting a measurement cycle in a satellite positioning system signal receiver.
2. Description of the Related Art
Global Positioning System (GPS) receivers use measurements from several satellites to compute position. GPS receivers normally determine their position by computing time delays between transmission and reception of signals transmitted from satellites and received by the receiver on or near the surface of the earth. The time delays multiplied by the speed of light provide the distance from the receiver to each of the satellites that are in view of the receiver.
More specifically, each GPS signal available for commercial use utilizes a direct sequence spreading signal defined by a unique pseudo-random noise (PN) code (referred to as the coarse acquisition (C/A) code) having a 1.023 MHz spread rate. Each PN code bi-phase modulates a 1575.42 MHz carrier signal (referred to as the L1 carrier) and uniquely identifies a particular satellite. The PN code sequence length is 1023 chips, corresponding to a one millisecond time period. One cycle of 1023 chips is called a PN frame or epoch.
GPS receivers determine the time delays between transmission and reception of the signals by comparing time shifts between the received PN code signal sequence and internally generated PN signal sequences. These measured time delays are referred to as “sub-millisecond pseudoranges”, since they are known modulo the 1 millisecond PN frame boundaries. By resolving the integer number of milliseconds associated with each delay to each satellite, then one has true, unambiguous, pseudoranges. A set of four pseudoranges together with a knowledge of absolute times of transmission of the GPS signals and satellite positions in relation to these absolute times is sufficient to solve for the position of the GPS receiver. The absolute times of transmission (or reception) are needed in order to determine the positions of the GPS satellites at the times of transmission and hence to compute the position of the GPS receiver.
Positioning systems, such as GPS, have fostered numerous applications that involve tracking people and assets. Various systems provide periodic location of a fixed asset, notification of proximity to pre-requested services, on-demand location identification, or continuous tracking of the location of a person or asset. Presently, such systems engage in satellite measurements at a device being tracked on a schedule unrelated to the relevance of the tracking information. This results in tracking the device continuously or tracking the device too infrequently to be effective. Continuous tracking directly results in increased power consumption in the device. Conversely, accessing the device too infrequently results in decreased accuracy and tracking performance.
Therefore, there exists a need in the art for a method and apparatus that provides for the automatic adjustment of a measurement cycle in a satellite positioning system signal receiver.